Neutron Laue and X-ray diffraction study of a new crystallographic superspace phase in n-nonadecane/urea
P. Rabiller1, S. Zerdane1, C. Mariette1, G.J. McIntyre2, M.-H. Lemée-Cailleau3, L. Guérin1, J.C. Ameline1, and B. Toudic1
1 Institut de Physique de
Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France
2 Australian Nuclear
Science and Technology Organisation, New Illawarra Road, Lucas Heights NSW 2234
Australia
3Institut
Laue-Langevin, 71 avenue des Martyrs, 38042 Grenoble cedex 9, France
philippe.rabiller@univ-rennes1.fr
Aperiodic composite crystals, such as n-alkane/urea inclusion compounds present long-range order without translation symmetry. For such host/guest [1] intergrowth nanotubular structures, which have a sole incommensurate direction c, a four-dimensional superspace [2-3] description usually gives the positions of the complete set of Bragg peaks. In such materials, symmetry breakings must be described as structural changes within crystallographic superspaces and the increase of the number of superspace groups with the increase of the dimension of the superspace allows many more structural solutions. We recently reported a sequence of phases in n-nonadecane/urea which involves at atmospheric pressure such structural degrees of freedom [4]. The study of the (T, P) phase diagram, through single crystal x-ray and neutron diffraction experiments, clearly evidenced the increase of structural solutions within this formalism [5]. With no evident reason at the time to suspect that further transitions would occur at lower temperature, these studies were limited to 80K. In fact this assumption was unfounded, and here we present a study down to 4K, making use of modern neutron Laue diffraction technique at atmospheric pressure, in which we evidence a supplementary phase transition within the crystallographic superspaces of dimension five.
Figure 1. Neutron Laue pattern obtained at the OPAL reactor (Australian Nuclear Science and Technology Organisation) evidencing a new phase below liquid N2 temperature in n-nonadecane/urea.